Aqueous coating composition

ABSTRACT

An aqueous coating composition constitutes an aqueous graft copolymer and an amino resin, the graft copolymer being obtained by neutralizing, with a base, the carboxyl groups of a copolymer which is obtained by copolymerizing the following components (a), (b), (c) and (d) and which contains 3-30% of component (a), 3-20% of component (b), 5-40% of component (c) and 10-89% of component (d): 
     (a) a radical-polymerizable polyester compound having a (meth)acryloyl group at one end, preferably a macromolecular monomer which is a copolymer of a hydroxyalkyl (meth)acrylate and another α,β-ethylenically unsaturated monomer, having a radical-polymerizable group at one end, 
     (b) an α,β-ethylenically unsaturated carboxylic acid, 
     (c) a hydroxyalkyl (meth)acrylate, and 
     (d) another vinyl monomer.

This is a division of parent application Ser. No. 08/202,564 filed Feb.28, 1994, now U.S. Pat. No. 5,455,306.

FIELD OF INVENTION

The present invention relates to an aqueous coating composition suitablefor coating of metals. The aqueous coating composition of the presentinvention can form a coating film having excellent resistancesparticularly to water at high temperatures and steam heating and istherefore suitable for use as a coating particularly for the outersurfaces of metal cans used for packaging of refreshing beverages,processed foods, etc.

BACKGROUND

Coatings for metal cans used for packaging of refreshing beverages,processed foods, etc. must have water resistance at high temperatures.As such coatings, there have heretofore been used organic solvent typecoatings capable of forming a coating film exhibiting excellentproperties when heat-cured, and examples thereof include epoxy/aminoresins, acrylic/amino resins and polyester/amino resins.

With the organic solvent type coatings, however, a large amount of anorganic solvent vaporizes into the air when they are applied, which hasinvited environmental pollution and a waste of resources.

Hence, aqueous coatings using water as a medium have been investigatedrecently. Conventional aqueous coatings, however, have low waterresistance. That is, their coating films show significant reduction inhardness and impact strength when subjected to a treatment withpressurized steam of 120° C. or higher (a retort treatment) and areunusable in practical applications, although said coating films areresistant to a treatment with hot water (e.g. boiling water).

In order to develop an aqueous coating resistant to the retorttreatment, Japanese Patent Application Kokai (Laid-Open) No. 72577/1991and U.S. Pat. No. 5,096,954 propose an aqueous coating compositioncomprising (a) an aqueous acrylic resin obtained by neutralizing, with abase, a copolymer which is composed mainly of an aromatic vinyl monomerand an alkyl (meth)acrylate and which contains carboxyl groups andhydroxyl groups in the molecule, (b) an amino resin and (c) a polyolwhich is a polyester, a polyether, a polyurethane or a polybutadieneeach having hydroxyl groups at the ends.

Further, EP 0526991A1 proposes an aqueous coating composition comprisingthe above resin (a), the above resin (b) and (d) a reaction product ofan amino compound and an epoxy resin.

Each of the above aqueous coating compositions employs a means forincreasing the crosslinking density of the aqueous acrylic resin inorder to obtain a cured coating film of high water resistance and usesthe resin (c) or (d) in order to obtain a tough coating film.

SUMMARY OF INVENTION

The first object of the present invention is to provide an aqueouscoating composition capable of forming a cured film resistant to a hotwater treatment and/or a hot steam treatment. The second object of thepresent invention is to provide a process for coating a metal substratewith said aqueous coating composition. The third object of the presentinvention is to provide a metal can for packaging of refreshingbeverages or processed foods, whose outer surface is coated with saidaqueous coating composition.

In order to achieve the above objects, the present inventors developedan aqueous coating composition comprising an aqueous graft copolymerhaving a particular constitution and an amino resin.

DETAILED DESCRIPTION OF EMBODIMENTS

The first aspect of the present invention lies in an aqueous coatingcomposition comprising an aqueous graft copolymer and an amino resin,said aqueous graft copolymer being obtained by neutralizing, with abase, the carboxyl groups of a copolymer which is obtained bycopolymerizing the following components (a), (b), (c) and (d) and whichcomprises a unit derived from the component (a), a unit derived from thecomponent (b), a unit derived from the component (c) and a unit derivedfrom the component (d) in amounts of 3-30% by weight, 3-20% by weight,5-40% by weight and 10-89% by weight, respectively, all based on thetotal of said units derived from the components (a) to (d):

(a) a radical-polymerizable compound selected from the group consistingof the following compounds (i), (ii) and (iii):

(i) a macromolecular monomer which is a (meth)-acrylonitrile/styrenecopolymer having a radical-polymerizable group at one end,

(ii) a macromolecular monomer which is a copolymer of a hydroxyalkyl(meth)acrylate and another α,β-ethylenically unsaturated monomer, havinga radical-polymerizable group at one end, and

(iii) a radical-polymerizable polyester which is a polyester of lowercondensation degree having a (meth)acryloyl group at one end,

(b) an α,β-ethylenically unsaturated carboxylic acid,

(c) a hydroxyalkyl (meth)acrylate, and

(d) another vinyl monomer.

The second aspect of the present invention lies in a process for coatinga metal substrate with the aqueous coating composition of the firstaspect.

The third aspect of the present invention lies in a metal can whoseouter surface is coated with said aqueous coating composition.

In the present invention, the radical-polymerizable compound (i) usableas the component (a) constituting the aqueous graft copolymer is amacromolecular monomer which is a (meth)acrylonitrile/styrene copolymerhaving a radical-polymerizable group at one end.

The radical-polymerizable group is preferably (meth)acryloyl group. Themacromolecular monomer has preferably a molecular weight of 2,000-30,000in terms of number-average molecular weight. Incidentally, in thepresent invention, the number-average molecular weight is apolystyrene-reduced number-average molecular weight measured by gelpermeation chromatography.

The (meth)acrylonitrile/styrene copolymer as the skeleton of themacromolecular monomer is preferably a (meth)acrylonitrile/styrenebinary copolymer. However, the skeleton copolymer may be a(meth)acrylonitrile/styrene type copolymer comprising, in addition tothe (meth)acrylonitrile and styrene monomer units, another monomer unitin an amount of 20% by weight or less based on the total constitutionalmonomer units of the copolymer.

When the skeleton of the macromolecular monomer is a(meth)acrylonitrile/styrene binary copolymer, the preferable proportionsof (meth)acrylonitrile and styrene in the binary copolymer are(meth)acrylonitrile/styrene =5-50% by weight/95-50% by weight.

The radical-polymerizable monomer usable, other than (meth)acrylonitrileand styrene includes, for example, styrene derivatives such asα-methylstyrene, p-methylstyrene and the like, and (meth)acrylic acidesters such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl(meth)acrylate, 2-ethylhexyl (meth)acrylate, cyclohexyl (meth)acrylateand the like.

In the present invention, the radical-polymerizable compound (ii) usableas the component (a) is a macromolecular monomer which is a copolymer ofa hydroxyalkyl (meth)acrylate and another α,β-ethylenically unsaturatedmonomer, having a radical-polymerizable group at one end.

The radical polymerizable group is preferably (meth)acryloyl group. Themacromolecular monomer has a molecular weight of preferably 2,000-30,000in terms of number-average molecular weight.

In the copolymer as the skeleton of the macromolecular monomer, theproportion of the hydroxyalkyl (meth)acrylate monomer unit is preferably5-40% by weight, more preferably 15-35% by weight based on the totalconstitutional monomer units of said copolymer.

When the proportion of the hydroxyalkyl (meth)acrylate monomer unit isless than 5% by weight, the resulting aqueous coating composition haslow water resistance and, when the proportion is more than 40% byweight, the aqueous coating composition gives a coating film of lowprocessability.

The hydroxyalkyl (meth)acrylate includes hydroxyethyl (meth)acrylate,2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate,hydroxybutyl (meth)acrylate, etc.

The other α,β-ethylenically unsaturated monomer constituting thecopolymer as the skeleton of the macromolecular monomer together withthe hydroxyalkyl (meth)acrylate, includes, for example, (meth)acrylicacid esters such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl(meth)acrylate, 2-ethylhexyl (meth)acrylate, cyclohexyl (meth)acrylateand the like; styrene derivatives such as styrene, α-methylstyrene,p-methylstyrene and the like; nitrile group-containing vinyl monomerssuch as (meth)acrylonitrile and the like; and alkylamino esters of(meth)acrylic acid.

The other α,β-ethylenically unsaturated monomer is preferably an alkyl(meth)acrylate having an alkyl moiety of 4-8 carbon atoms, in view ofthe water resistance and weatherability of the coating film formed withthe resulting aqueous coating composition. The preferable amount of saidalkyl (meth)acrylate monomer unit in the copolymer as the skeleton ofthe macromolecular monomer is 50-95% by weight based on the totalconstitutional monomer units of said copolymer.

In the present invention, the radical-polymerizable compound (iii)usable as the component (a) is a radical-polymerizable polyester whichis a polyester of low condensation degree having a (meth)acryloyl groupat one end. Preferable examples thereof are compounds represented by thefollowing chemical formulas (1) and (2), and the compound represented bychemical formula (1) is more preferable: ##STR1## wherein R¹ is hydrogenatom or methyl group, R² is an alkylene group of 2-4 carbon atoms, and nis 2-10, and ##STR2## wherein R¹ is hydrogen atom or methyl group, R² isan alkylene group of 17 carbon atoms, and n is 2-10.

The radical-polymerizable polyester has a number-average molecularweight of preferably 400-4,000. When the number-average molecular weightis smaller than 400, the resulting aqueous coating composition gives acoating film which easily reduces its impact strength after a retorttreatment. When the molecular weight is larger than 4,000, the aqueouscoating composition hardly gives a coating film of high hardness.

The compound represented by chemical formula (1) can be synthesized bysubjecting ε-caprolactone to ring-opening polycondensation in thepresence of hydroxyethyl (meth)acrylate using a catalyst such asstannous chloride or the like [e.g. Japanese Patent Application Kokai(Laid-Open) No. 195714/1982]. As the compound represented by chemicalformula (1), there may be used commercial products such as Placcel FMseries (caprolactone-modified methacrylic acid esters), i.e., FM-1,FM-2, FM-3, FM-4, FM-5, etc. and Placcel FA series(caprolactone-modified acrylic acid esters), i.e., FA-1, FA-2, FA-3,FA-4, FA-5, etc, all of which are products of DAICEL CHEMICALINDUSTRIES, LTD.

The compound represented by chemical formula (2) can be synthesized bypolycondensing 12-hydroxystearic acid in the presence of an acidcatalyst, to obtain a polyester of low condensation degree having ahydroxyl group and a carboxyl group at the molecular ends and thenreacting the polyester with glycidyl (meth)acrylate.

The aqueous graft copolymer of the present invention can be obtained bycopolymerizing the radical-polymerizable compound (a) selected from thegroup consisting of the above-mentioned compounds (i) to (iii), with thecomponents (b) to (d) described below in detail, to obtain acarboxyl-containing copolymer (hereinafter referred to simply ascopolymer) and then neutralizing the copolymer with a base. In thecopolymer, the proportion of the unit derived from the component (a) is,as mentioned previously, 3-30% by weight, preferably 5-20% by weightbased on the total monomer units of the copolymer.

When the proportion of the unit derived from the component (a) in thecopolymer is less than 3% by weight, the aqueous graft copolymerobtained by neutralizing the copolymer has a low impact strength and,when the proportion is more than 30% by weight, the aqueous graftcopolymer has a low hardness.

Next, the α,β-ethylenically unsaturated carboxylic acid, which is thecomponent (b), includes acrylic acid, methacrylic acid, crotonic acid,maleic acid, maleic anhydride, fumaric acid, itaconic acid, etc. Amongthem, acrylic acid and methacrylic acid are preferred.

When the proportion of the unit derived from the component (b) in thecopolymer is less than 3% by weight, the copolymer is insufficientlywater-soluble or water-dispersible. When the proportion is more than 20%by weight, the resulting aqueous coating composition gives a coatingfilm of low water resistance.

The hydroxyalkyl (meth)acrylate, which is the component (c), includeshydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate,3-hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, etc.

When the proportion of the unit derived from the component (c) in thecopolymer is less than 5% by weight, the resulting aqueous coatingcomposition gives a coating film of low water resistance. When theproportion is more than 40% by weight, the aqueous coating compositiongives a coating film of low impact resistance. The preferable proportionof the unit derived from the component (c) is 5-30% by weight.

The other vinyl monomer [the component (d)] constituting the copolymerof the present invention together with the components (a) to (c)includes, for example, (meth)acrylic acid esters such as methyl(meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl(meth)acrylate, cyclohexyl (meth)acrylate and the like; styrenederivatives such as styrene, α-methylstyrene, p-methylstyrene and thelike; (meth)acrylonitrile; (meth)acrylamide;N-methoxymethyl-(meth)acrylamide; N-butoxymethyl(meth)acrylamide; andalkylamino esters of (meth)acrylic acid. These monomers can be usedsingly or in combination of two or more.

Among the other vinyl monomers mentioned above, monomers having ahydroxyl group--reactive functional group, such asN-methoxymethyl(meth)acrylamide, N-(n-butoxy)methyl(meth)acrylamide andthe like are preferred, because the resulting aqueous coatingcomposition can give a coating film of higher crosslinking density. Theunit of such a monomer having the functional group in the copolymer ispreferably not more than 30% by weight.

The proportion of the unit derived from the component (d) in thecopolymer is a value obtained by deducting the proportion of the unitsderived from the components (a) to (c), from the total monomer units(i.e. 100% by weight) of the copolymer and is 10-89% by weight,preferably 40-70% by weight, more preferably 60-70% by weight.

The copolymer composed of the units derived from the components (a) to(d) has a hydroxyl value of preferably 20-150 KOH mg/g, more preferably30-100 KOH mg/g. When the copolymer has a hydroxyl value of less than 20KOH mg/g, the resulting aqueous coating composition has inferiorprocessability. When the copolymer has a hydroxyl value of more than 150KOH mg/g, the aqueous coating composition gives a coating film of lowwater resistance.

Since each of the component monomers (a) to (d) can be polymerized at aconversion of substantially 100% by a known radical polymerizationprocess, the monomers (a) to (d) are fed in proportions corresponding tothose of the monomer units in the resulting copolymer and copolymerized,whereby a copolymer having the above constitution can be obtained.

The outline of the preferable process for production of the copolymercomposed of the monomers (a) to (d) is as follows.

That is, the monomers (a) to (d) are radical-polymerized at 60°-150° C.in the presence of a radical polymerization initiator such as2,2'-azobisisobutyronitrile or the like, using a polymerization solventsuch as isopropyl alcohol, n-butanol, isobutyl alcohol, ethylene glycolmonomethyl ether, ethylene glycol monoethyl ether or the like.

In the above polymerization, use of an appropriate amount of a chaintransfer agent such as n-dodecylmercaptan, mercaptoacetic acid,thiomalic acid, mercaptoethanol, mercaptopropionic acid or the like canallow the resulting copolymer to have a controlled molecular weight.

In the present invention, the molecular weight of the copolymer ispreferably 2,000-20,000, more preferably 3,000-15,000 in terms ofnumber-average molecular weight.

Thereafter, part or all of the carboxyl groups of the copolymer areneutralized with a base. Preferably, all the carboxyl groups areneutralized. By the neutralization, the copolymer is converted into awater-soluble or water-dispersible copolymer.

The neutralization can be conducted by a simple procedure of adding anaqueous base solution to an organic solvent solution of the copolymer,obtained by the above-mentioned polymerization.

As the base, there can be used, for example, organic amines such asmonoethanolamine, dimethylamine, diethylamine, triethylamine,triethanolamine, diethylethanolamine, dimethylethanolamine and the like,as well as ammonia.

When the aqueous coating composition of the present invention is used asa coating for metal cans, dimethylethanolamine is preferred as the base.When the aqueous coating composition is used as a coating for metalsubstrates other than metal cans, triethylamine is preferred as thebase.

The aqueous graft copolymer solution obtained by the above procedure isthen subjected to distillation under reduced pressure to remove theorganic solvent used as a polymerization solvent. The resulting materialis mixed with water to obtain an aqueous solution or dispersion of thegraft copolymer, having a solid content of 20-70% by weight, which issuitable for coating of a metal substrate.

In the present invention, the amino resin used in combination with theabove-obtained aqueous graft copolymer includes, for example,alkyl-etherified melamine resins such as methyl-etherified melamineresin, butyl-etherified melamine resin and the like; alkyl-etherifiedurea resins; and alkyl-etherified benzoguanamine resins such asmethyl-etherified benzoguanamine resin, ethyl-etherified benzoguanamineresin and the like. These amino resins can be used singly or incombination of two or more.

Among them, alkyl-etherified benzoguanamine resins such asmethyl-etherified benzoguanamine resin, ethyl-etherified benzoguanamineresin and the like are preferred.

With respect to the degree of alkyl etherification of the amino resin,there can be used any of those amino resins wherein part or all of theactive hydrogen atoms of amino group have been replaced by alkyl ethergroup(s).

The preferable amount of the amino resin used is 10-80 parts by weightper 100 parts by weight of the solid content of the aqueous graftcopolymer.

The aqueous coating composition of the present invention may comprisevarious additives for coatings, in addition to the aqueous graftcopolymer and the amino resin. Some examples of the additives are afilm-forming assistant (e.g., ethylene glycol monobutyl ether) and alevelling agent.

With the aqueous coating composition of the present invention, it ispossible to form, on a metal substrate, a cured coating film superior inhardness, water resistance, adhesivity and gloss. The present aqueouscoating composition is suitable as a coating particularly for the outersurface of a metal can used for packaging of refreshing beverages orprocessed foods.

The aqueous solution or dispersion of the aqueous graft copolymer ismixed with the amino resin in the above-mentioned proportions; themixture is coated on a metal substrate such as aluminum, stainlesssteel, zinc-treated steel plate, any other treated steel plate, tinplate or the like, followed by curing at 150°-220° C. for about 1-20minutes; thereby, a cured coating film can be formed.

The aqueous coating composition of the present invention can be coateddirectly on a metal, or may be coated on an ink or a primer.

The present invention is hereinafter described more specifically withreference to Examples and Comparative Examples.

The details of the macromolecular monomers, etc. used in Examples andComparative Examples are as follows.

Macromolecular monomer AN-6

A macromolecular monomer of poly(acrylonitrile/styrene) type produced byTOAGOSEI CHEMICAL INDUSTRY CO., LTD., having a methacryloyl group as apolymerizable group, a number-average molecular weight of 6,000 andproportions of acrylonitrile monomer unit/styrene monomer unit =25% byweight/75% by weight.

Macromolecular monomer AA-6

A macromolecular monomer of polymethyl methacrylate type produced byTOAGOSEI CHEMICAL INDUSTRY CO., LTD., having a methacryloyl group as apolymerizable group and a number-average molecular weight of 6,000.

Placcel FM-5

A radical-polymerizable polyester produced by DAICEL CHEMICALINDUSTRIES, LTD., a monomer having a number-average molecular weight of700 which is a caprolactone oligomer having a methacryloyl group at oneend.

Placcel FM-3

A radical-polymerizable polyester produced by DAICEL CHEMICALINDUSTRIES, LTD., a monomer having a number-average molecular weight of470 which is a caprolactone oligomer having a methacryloyl group at oneend.

Placcel FM-8

A radical-polymerizable polyester produced by DAICEL CHEMICALINDUSTRIES, LTD., a monomer having a number-average molecular weight of1,040 which is a caprolactone oligomer having a methacryloyl group atone end.

REFERENTIAL EXAMPLE 1 (Synthesis of macromolecular monomer M-1)

4.9 g of cyclohexyl methacrylate, 55.0 g of 2-ethylhexyl methacrylateand 25.1 g of 2-hydroxyethyl methacrylate were mixed. One third of theresulting mixture, 10.5 g of styrene, 0.9 g of mercaptopropionic acid asa chain transfer agent and 30 g of toluene were fed into a glass flaskequipped with a stirrer, two dropping funnels, a gas-introducing tubeand a thermometer. The contents in the flask were heated to 90° C.

From one of the two dropping funnels were dropped, in 3 hours, theremainder (two thirds) of the above mixture, 4.5 g of styrene and 2.1 gof mercaptopropionic acid. Simultaneously therewith, 0.3 g ofazobisisobutyronitrile dissolved in 35.0 g of toluene was dropped fromthe other dropping funnel in 3 hours.

Then, 0.3 g of azobisisobutyronitrile dissolved in 35.0 g of toluene wasdropped in 2 hours. The resulting mixture was subjected to a furtherreaction of hours to obtain a solution of a polymer having a carboxylgroup at one end.

To the solution of a polymer having a carboxyl group at one end wereadded 0.04 g of methoxybenzoquinone, 2.7 g of tetrabutylammonium bromideand 2.2 g of glycidyl methacrylate. The mixture was subjected to areaction at 90° C. for 5 hours with air being blown thereinto, to obtaina macromolecular monomer M-1 having a methacryloyl group at one end at apurity of 99.7% as calculated from the decrease of the acid value of thepolymer. The macromolecular monomer M-1 had a polystyrene-reducednumber-average molecular weight of 3,300 as measured by gel permeationchromatography. In the macromolecular monomer M-1, the monomercomposition of the polymer skeleton was cyclohexylmethacrylate/2-ethylhexyl methacrylate/hydroxyethyl methacrylate/styrene=4.9/55/25.1/15 (by weight %).

REFERENTIAL EXAMPLE 2 (Synthesis of macromolecular monomer M-2)

The procedure of Referential Example 1 was repeated except that thetotal amount of mercaptopropionic acid (chain transfer agent) used waschanged to 0.79 g, to obtain a macromolecular monomer M-2 having amethacryloyl group at one end at a purity of 99.6%, having anumber-average molecular weight of 6,000.

REFERENTIAL EXAMPLE 3 (Synthesis of aqueous graft copolymer A)

The following monomers and 13.5 g of styrene were copolymerized.Incidentally, the following monomers were mixed and used as a mixture.

    ______________________________________                                        AN-6 [macromolecular monomer of                                                                          10 g                                               poly(acrylonitrile/styrene) type]                                             n-Butyl methacrylate (hereinafter                                                                        11.6 g                                             referred to as nBMA)                                                          n-Butyl acrylate (hereinafter referred                                                                   46.8 g                                             to as nBA)                                                                    Acrylic acid (hereinafter referred to                                                                    5.2 g                                              as AA)                                                                        2-Hydroxyethyl methacrylate (hereinafter                                                                 12.9 g                                             referred to as HEMA)                                                          ______________________________________                                    

That is, one third of the mixture of the above monomers, 9.5 g ofstyrene, 0.18 g of mercaptoethanol, 6.3 g of ethylene glycol monobutylether and 14.7 g of isopropyl alcohol were fed into a glass flaskequipped with a stirrer, a reflux condenser, two dropping funnels, agas-introducing tube and a thermometer. The contents in the flask wereheated to 87° C.

From one of the two dropping funnels was dropped, in 3 hours, a mixtureof the remainder (two thirds) of the monomer mixture, 4.0 g of styreneand 0.27 g of mercaptoethanol. Simultaneously therewith, apolymerization initiator solution consisting of 19.2 g of ethyleneglycol monobutyl ether, 44.8 g of isopropyl alcohol and 0.2 g of2,2'-azobis(2-methylbutyronitrile) (hereinafter referred to as ABN-E)was dropped from the other dropping funnel in 3 hours.

Then, 4.5 g of ethylene glycol monobutyl ether, 10.5 g of isopropylalcohol and 0.46 g of ABN-E were dropped in 2 hours. Thereafter,stirring was conducted for 2 hours to synthesize a carboxylgroup-containing copolymer. The copolymer had a polystyrene-reducednumber-average molecular weight of 8,000 as measured by gel permeationchromatography.

The above-obtained solution of a carboxyl group-containing copolymer washeated to 40° C. and subjected to distillation under reduced pressure toremove isopropyl alcohol. The resulting material was mixed with 6.4 g ofdimethylethanolamine (hereinafter referred to as DMEA) and 70 g ofdistilled water to conduct neutralization.

By the above procedure was obtained a semitransparent viscous aqueoussolution of a graft copolymer A, having a solid content of 50% by weightand containing 15% by weight of ethylene glycol monobutyl ether.

REFERENTIAL EXAMPLES 4-6 (Synthesis of aqueous graft copolymers B, C andD)

The components shown in Table 1 were polymerized in the same manner asin Referential Example 3, followed by neutralization with DMEA, toobtain aqueous solutions of graft copolymers B, C and D.

                  TABLE 1                                                         ______________________________________                                                   Aqueous copolymer                                                             A      B        C        D                                         ______________________________________                                        AN-6       (g)   10.0     20.0   --     --                                    AA-6       (g)   --       --     10.0   --                                    nBMA       (g)   11.6     1.7    --     12.8                                  MMA        (g)   --       --     13.2   --                                    St         (g)   13.5     12.0   13.2   15.0                                  EA         (g)   --       --     5.4    --                                    nBA        (g)   46.8     49.6   40.5   52.0                                  AA         (g)   5.2      5.2    5.2    5.8                                   HEMA       (g)   12.9     11.5   12.5   14.4                                  (Total of above)                                                                         (g)   (100)    (100)  (100)  (100)                                 DMEA       (g)   6.4      6.5    6.4    7.2                                   ______________________________________                                         Note:                                                                         MMA refers to methyl methacrylate; St refers to styrene; and EA refers to     ethyl acrylate.                                                          

EXAMPLES 1-3

Each of the aqueous graft copolymer solutions obtained in ReferentialExamples 3-6 was mixed with 40 parts by weight, per 100 parts by weightof the solid content of the copolymer, of an amino resin [Cymel 1123(benzoguanamine resin) produced by Mitsui Cyanamid Ltd. or Cymel 303(hexamethoxymethylolmelamine) produced by Mitsui Cyanamid Ltd.] in acombination shown in Table 2, to obtain aqueous coating compositions.

In Table 2, the curing agent H1 refers to Cymel 1123 and the curingagent H2 refers to Cymel 303. The same applies also to Table 4 whichappears later.

To each of the aqueous coating compositions were added a silicone typelevelling agent, ethylene glycol monobutyl ether and water so as to havean organic solvent content of 15% by weight and a solid content of 30%by weight. Each of the resulting coating compositions was coated on analuminum plate using a bar coater, in a film thickness of 5-6 μm. Theformed coating film was cured at 200° C. for 10 minutes.

Each of the cured coating films obtained above and the coating filmsafter having been subjected to a pressurized steam treatment (30 minutesstanding in steam of 130° C.) using a pressure cooker apparatus, wasmeasured for various properties. The results are shown in Table 2.

COMPARATIVE EXAMPLES 1-3

Aqueous coating compositions were produced in the same manner as inExample 1, using aqueous graft copolymers and amino resins shown inTable 2. Coating films were formed with the aqueous coating compositionsand measured for properties. The results are shown in Table 2.

The comparison of Examples 1-3 with Comparative Examples 1-3 indicatethat the cured coating films formed using the aqueous copolymers C or Dshowed very poor properties after having been subjected to thepressurized steam treatment.

                                      TABLE 2                                     __________________________________________________________________________               Example     Comparative Example                                               1   2   3   1   2   3                                              __________________________________________________________________________    Aqueous copolymer                                                                        A   A   B   C   D   D                                              Curing agent                                                                             H1  H2  H1  H2  H1  H2                                             Varnish appearance                                                                       Trans-                                                                            Trans-                                                                            Trans-                                                                            Trans-                                                                            Trans-                                                                            Trans-                                                    parent                                                                            parent                                                                            parent                                                                            parent                                                                            parent                                                                            parent                                         Results (before treat-                                                        ment with pressurized                                                         steam)                                                                        Impact resistance (cm)                                                                   45  40  35  30  30  20                                             Water resistance                                                                         Pass                                                                              Pass                                                                              Pass                                                                              Pass                                                                              Pass                                                                              Partially                                                                     whitened                                       Adhesivity (%)                                                                           100 100 100 100 100 100                                            Hardness   F   H   H   H   HB  F                                              Results (after treat-                                                         ment with pressurized                                                         steam)                                                                        Impact resistance (cm)                                                                   35  15  30  10  5   0                                              Adhesivity (%)                                                                           100 100 100 100 100 100                                            Hardness   F   F   H   F   HB  F                                              __________________________________________________________________________

In Table 2, the properties of coating film were all measured inaccordance with the test methods specified by JIS K 5400.

Impact resistance

Measured by a DuPont type impact test (point of impact =1/2 in.; load=500 g).

Water resistance

Measured by a boiling water resistance test (time for sample dipping =60minutes).

Adhesivity

Measured by a tape cross-cutting test (evaluation was made based on thedegree of remaining coating film after peeling of tape).

Hardness

Measured by a pencil scratching test.

REFERENTIAL EXAMPLES 7-9 (Synthesis of aqueous graft copolymers E, F andG)

Aqueous graft copolymers E, F and G having the monomer compositionsshown in Table 3 were produced using the macromolecular monomer M-1 orM-2 having a HEMA monomer unit.

                  TABLE 3                                                         ______________________________________                                                     Aqueous copolymer                                                             E        E       G                                               ______________________________________                                        M-1          (g)   10.0       --    --                                        M-2          (g)   --         10.0  10.0                                      nBMA         (g)   --         --    11.8                                      MMA          (g)   1.8        1.8   --                                        St           (g)   11.0       11.0  11.0                                      EA           (g)   21.0       21.0  --                                        nBA          (g)   38.0       38.0  49.0                                      AA           (g)   5.2        5.2   5.2                                       HEMA         (g)   13.0       13.0  13.0                                      (total of above)                                                                           (g)   (100)      (100) (100)                                     DMEA         (g)   6.4        6.4   6.4                                       ______________________________________                                    

EXAMPLES 4-8

Using the aqueous graft copolymers E, F and G obtained in ReferentialExamples 7-9, coating films were formed in the same manner as inExamples 1-3. The coating films were measured for properties. Theresults are shown in Table 4.

                  TABLE 4                                                         ______________________________________                                                   Example                                                                       4     5       6       7     8                                      ______________________________________                                        Aqueous copolymer                                                                          E       E       F     G     G                                    Curing agent H1      H2      H1    H1    H2                                   Varnish appearance                                                                         Trans-  Trans-  Trans-                                                                              Trans-                                                                              Trans-                                            parent  parent  parent                                                                              parent                                                                              parent                               Results (before treat-                                                        ment with pressurized                                                         steam)                                                                        Impact resistance (cm)                                                                     45      40      45    45    40                                   Water resistance                                                                           Pass    Pass    Pass  Pass  Pass                                 Adhesivity (%)                                                                             100     100     100   100   100                                  Hardness     F       H       F     F     H                                    Results (after treat-                                                         ment with pressurized                                                         steam)                                                                        Impact resistance (cm)                                                                     35      15      30    35    15                                   Adhesivity (%)                                                                             100     100     100   100   100                                  Hardness     F       F       F     H     F                                    ______________________________________                                    

REFERENTIAL EXAMPLES 10-14 (Synthesis of aqueous graft copolymers H toL)

Aqueous graft copolymers H to L were synthesized in the same manner asin Referential Example 3, using the monomers shown in Table 5.

                  TABLE 5                                                         ______________________________________                                                   Aqueous copolymer                                                             H     I       J       K     L                                      ______________________________________                                        Placcel FM5                                                                              (g)   20.0    10.0  --    --    --                                 Placcel FM3                                                                              (g)   --      --    20.0  --    --                                 Placcel FM8                                                                              (g)   --      --    --    20.0  --                                 EHMA       (g)   48.3    --    --    --    --                                 iBMA       (g)   --      20.5  17.9  26.7  12.5                               St         (g)   14.4    22.4  20.0  20.0  15.0                               nBA        (g)   --      27.9  24.8  16.0  52.5                               AA         (g)   5.8     5.8   5.8   5.8   5.8                                HEMA       (g)   11.5    13.4  11.5  11.5  14.4                               (Total of above)                                                                         (g)   (100)   (100) (100) (100) (100)                              DMEA       (g)   6.4     6.4   7.2   7.2   7.2                                ______________________________________                                         Note:                                                                         EHMA and iBMA refer to 2ethylhexyl methacrylate and isobutyl methacrylate     respectively.                                                            

EXAMPLES 9-12 AND COMPARATIVE EXAMPLE 4

Using the aqueous graft copolymers H to L obtained in ReferentialExamples 10-14 and Cymel 1123, coating films were formed in the samemanner as in Examples 1-3. The coating films were measured forproperties. The results are shown in Table 6.

                  TABLE 6                                                         ______________________________________                                                Example           Comparative                                                 9     10      11      12    Example 4                                 ______________________________________                                        Aqueous   H       I       J     K     L                                       copolymer                                                                     Varnish   Trans-  Trans-  Trans-                                                                              Trans-                                                                              Transparent                             appearance                                                                              parent  parent  parent                                                                              parent                                        Results (before                                                               treatment with                                                                pressurized                                                                   steam)                                                                        Impact    45      40      40    45    30                                      resistance (cm)                                                               Water     Pass    Pass    Pass  Pass  Pass                                    resistance                                                                    Adhesivity (%)                                                                          100     100     100   100   100                                     Hardness  F       H       H     F     HB                                      Results (after                                                                treatment with                                                                pressurized                                                                   steam)                                                                        Impact    40      35      35    40    5                                       resistance (cm)                                                               Adhesivity (%)                                                                          100     100     100   100   100                                     Hardness  H       H       F     F     HB                                      ______________________________________                                    

We claim:
 1. An aqueous coating composition comprising an aqueous graftcopolymer and an amino resin, said aqueous graft copolymer beingobtained by neutralizing, with a base, the carboxyl groups of acopolymer which is obtained by copolymerizing the following components(a), (b), (c) and (d) and which comprises a unit derived from thecomponent (a), a unit derived from the component (b), a unit derivedfrom the component (c) and a unit derived from the component (d) inamounts of 3-30% by weight, 3-20% by weight, 5-40% by weight and 10-89%by weight, respectively, all based on the total of said units derivedfrom the components (a) to (d):(a) a radical-polymerizable polyesterwhich is a polyester of lower condensation degree having a(meth)acryloyl group at one end, (b) an α,β-ethylenically unsaturatedcarboxylic acid, (c) a hydroxyalkyl (meth)acrylate, and (d) anothervinyl monomer.
 2. A composition according to claim 1, wherein theradical-polymerizable polyester is represented by the following chemicalformula (1) or (2) and has a number-average molecular weight of400-4,000: ##STR3## wherein R¹ is hydrogen atom or methyl group, R² isan alkylene group of 2-4 carbon atoms, and n is 2-10, or ##STR4##wherein R¹ is hydrogen atom or methyl group, R² is an alkylene group of17 carbon atoms, and n is 2-10.
 3. A composition according to Claim 1,wherein the α,β-ethylenically unsaturated carboxylic acid as thecomponent (b) is selected from the group consisting of acrylic acid,methacrylic acid, crotonic acid, maleic acid, maleic anhydride, fumaricacid and itaconic acid.
 4. A composition according to claim 1, whereinthe hydroxyalkyl (meth)acrylate as the component (c) is selected fromthe group consisting of hydroxyethyl (meth)acrylate, 2-hydroxypropyl(meth)acrylate, 3-hydroxypropyl (meth)acrylate and hydroxybutyl(meth)acrylate.
 5. A composition according to claim 1, wherein the othervinyl monomer as the component (d) is selected from the group consistingof (meth)acrylic acid esters, styrene derivatives, (meth)acrylonitrile,(meth)acrylamide, N-methoxymethyl(meth)acrylamide,N-butoxymethyl(meth)acrylamide and alkylamino esters of (meth)acrylicacid.
 6. A composition according to claim 5, wherein the other vinylmonomer as the component (d) is a monomer having a hydroxylgroup--reactive functional group, consisting ofN-methoxymethyl(meth)acrylamide or N-(n-butoxy)methyl(meth)acrylamide ora mixture thereof, which is contained in the copolymer in an amount of30% by weight or less.
 7. A composition according to claim 1, whereinthe copolymer composed of the components (a) to (d) has a hydroxyl valueof 20-150 KOH mg/g and a number-average molecular weight of2,000-20,000.
 8. A composition according to claim 1, wherein the base isselected from the group consisting of monoethanolamine, dimethylamine,diethylamine, triethylamine, triethanolamine, diethylethanolamine,dimethylethanolamine and ammonia.
 9. A composition according to claim 1,wherein the amino resin is selected from the group consisting ofalkyl-etherified melamine resins, alkyl-etherified urea resins andalkyl-etherified benzoguanamine resins.
 10. A composition according toclaim 1, wherein the amino resin is used in an amount of 10-80 parts byweight per 100 parts by weight, in terms of solid content, of theaqueous graft copolymer.
 11. A process for coating a metal substrate,which comprises coating the metal substrate with an aqueous coatingcomposition according to claim
 1. 12. A metal can whose outer surface iscoated with an aqueous coating composition according to claim
 1. 13. Anaqueous coating composition according to claim 1 wherein said copolymercomprises 15-35% by weight of said hydroxyalkyl (meth)acrylate based onthe total constitutional monomer units of said copolymer.
 14. Acomposition according to claim 1 wherein said unit derived fromcomponent (a) comprises 5-20% by weight, said unit derived fromcomponent (c) comprises 5-30% by weight, and said unit derived fromcomponent (d) comprises 60-70% by weight, all based on the weight ofsaid aqueous graft copolymer; said aqueous graft copolymer has anumber-average molecular weight of 3,000-15,000; and the solids contentof said aqueous graft copolymer in said aqueous coating composition is20-70% by weight.